The present invention relates to a welding gun attached to an arm of a welding robot and provided with a backup cylinder.
It is conventional that an equalizing device is provided between a robot arm and a welding gun.
As illustrated in FIG. 4, an equalizing device 63 is placed between a robot arm 61 and a welding gun 62. The welding gun 62 comprises a fixed arm 65 having an electrode 64 at the tip end thereof, a movable arm 67 having an electrode 66 at the tip end thereof and a cylinder 68 fixed to the fixed arm 65 for moving the movable arm 67 toward or away from the electrode 64. With such an arrangement, a workpiece 69 is positioned between the fixed arm 65 and the movable arm 67 so that the electrodes 64, 66 are movable toward the workpiece 69 by movement of the welding gun 62 by the robot arm 61, thereafter the cylinder 68 is actuated so that the electrode 66 provided at the tip end of the movable arm 67 is brought into contact with the workpiece 69. At this state, the movable arm 67 is liable to further move forward, but the forward movement thereof acts against the fixed arm 65 as the reaction force since the electrode 66 is brought into contact with the workpiece 69. Hence the fixed arm 65 is moved by the equalizing device 63 so that the electrode 64 is brought into contact with the opposite surface of the workpiece 69. When both of the electrodes 66, 64 are brought into contact with the workpiece 69, the pressure force is further applied to the workpiece 69 and the current is applied to the workpiece 69 so that the workpiece 69 is welded.
The prior art welding gun has, however, a drawback that distortion is apt to occur at one surface of the workpiece 69 caused by the electrode 64 since the inertia becomes large if the fixed arm 65 is large at the time when the fixed arm 65 is moved by the reaction force set forth above. This provides an obstacle to a successful welding operation.